This invention relates to a reflector circuit for receiving illuminating radiation and emitting corresponding amplified output radiation in response; the invention is particularly, although not exclusively, concerned with a reflector circuit for use in a pseudo passive transponder (PPT) tag.
One type of reflector circuit known in the prior art, namely a conventional PPT, uses a diode detector to detect incoming radiation from an interrogating source. The diode can be operated as a modulated reflector by modulating a bias potential applied thereto with an information carrying signal such that the PPT reflects the incoming radiation back to the source as modulated reflected radiation. Since the conventional PPT does not incorporate an amplifier in association with the diode, the reflected radiation is of reduced amplitude with respect to the incoming radiation; this limits a useful range over which the PPT can respond for a given radiation output power from the interrogating source, namely carrier radiation power emitted from the source. The PPT responds to the incoming radiation by emitting corresponding reflected radiation with an efficiency referred to as its xe2x80x9cconversion efficiencyxe2x80x9d. This conversion efficiency is defined as a ratio of the carrier radiation power received by the PPT to sideband radiation power of the corresponding reflected radiation emitted from the PPT. For the conventional PPT described above, the conversion efficiency is typically xe2x88x928dB or less which necessitates use of a radiation output power from the interrogating source of tens of milliwatts to achieve a useful operating range from the source to the PPT of a few meters. Such a short operating range is undesirable in many applications. It is often impermissible to increase the radiation output power from the source, for example for safety reasons. Moreover, general background radiation giving rise to receiver noise at the source limits its sensitivity for detecting reflected radiation from the conventional PPT.
Another type of reflector circuit, namely a pseudo continuous wave radar transponder as described in a UK patent number GB 2 051 522A, incorporates an antenna assembly and a transmission radio frequency (r.f.) amplifier for enhancing its conversion efficiency and thereby providing it with an extended operating range. Since it is difficult to prevent an r.f. amplifier coupled to an antenna assembly for receiving incoming radiation and emitting corresponding amplified output radiation from spontaneously oscillating, the pseudo transponder additionally incorporates a delay line and associated switches controlled from a clock generator to counteract spontaneous oscillations. Spontaneous oscillation is defined as unwanted oscillations occurring within a signal path providing amplification by virtue of residual feedback arising around the path. Incoming radiation is received at the antenna assembly and converted thereat to a received signal which is then sampled by one of the switches, amplified by the amplifier, stored in the delay line for a period of time, further amplified by the amplifier before finally being emitted as reflected radiation from the antenna assembly. Incorporation of the switches and delay line assists to counteract spontaneous oscillations arising in the amplifier.
The inventor of the present invention has appreciated that a problem arises when the pseudo transponder described in the foregoing is simultaneously interrogated by several sources operating at mutually different radiation emission frequencies, one of which provides sufficient incoming radiation at the transponder to cause its r.f. amplifier to obscure or distort signals transmitted therethrough, namely to cause overload in the amplifier or cause generation of intermodulation artefacts. Such overload can obscure relatively weaker incoming radiation received at the transponder which is itself insufficiently powerful to cause overload in the transponder, thereby potentially preventing the pseudo transponder from responding to the weaker radiation.
A conventional solution to the problem above is to incorporate a gain control in association with the r.f. amplifier to reduce its amplification when overload occurs. When relatively weaker signals are received by the pseudo transponder, the control is arranged so that the amplifier transmits and amplifies signals by a nominal gain. When received radiation at the transponder is sufficiently powerful to cause overload within the amplifier when providing its nominal gain, the control is arranged to reduce amplifier gain to counteract such overload. Such gain reduction is undesirable because it reduces amplification proportionately at all frequencies to which the transponder is responsive. Thus, weaker radiation received at the transponder at a first frequency is amplified with less than the nominal gain when stronger radiation at a second frequency is simultaneously received thereat which causes the gain control to reduce gain provided by the r.f. amplifier.
The conventional solution is unsatisfactory when the transponder is interrogated simultaneously by several sources, one of which is remote and provides weaker radiation at the transponder and other of which is close and provides stronger radiation thereat sufficiently powerful to cause amplifier overload when providing nominal gain. It is desirable that the transponder responds to the weaker radiation and the stronger radiation using the nominal gain and reduced gain respectively.
According to the present invention, there is provided a reflector circuit for receiving illuminating radiation and emitting corresponding amplified output radiation, the circuit comprising
an antenna assembly for receiving the illuminating radiation and providing a corresponding received signal, and
processing means for amplifying and storing a portion of the received signal for a period of time for use in generating a corresponding output signal for emission from the antenna assembly as the output radiation, in which the processing means is arranged to provide frequency selective amplification in response to the magnitude of components present in the input radiation.
This provides the advantage that the circuit is capable of modifying its response at frequencies where the illuminating radiation is sufficiently powerful to cause overload, thereby counteracting overload at those frequencies whilst providing an unmodified response to illuminating radiation at frequencies where there is insufficient illuminating radiation power received at the circuit to cause overload therein.
If the circuit responds non-progressively when the magnitude of components in the illuminating radiation exceeds a threshold power level at which the circuit selectively modifies its response to counteract overload or provide compression, a problem arises. The circuit can therefore be arranged to provide amplification which progressively reduces in response to increased magnitude of components in the illuminating radiation. This provides an advantage that spurious circuit response is less likely to occur when the magnitude of components in the illuminating radiation is substantially similar to the threshold power level.
In one embodiment of the invention, the processing means can incorporate storing means for storing the portion of the signal for use in generating the output signal, the storing means incorporating a magnetostatic wave device arranged to provide a frequency selective response. This provides an advantage that the device simultaneously provides a storing function and a frequency selective response, thereby providing a simplified reflector circuit. Operation of magnetostatic wave devices will be further described later.
Advantageously, the magnetostatic device provides a signal propagation path through an epitaxial Yttrium Iron Garnet magnetic film having a thickness in a range of 10 xcexcm to 100 xcexcm for storing the portion of the signal and providing the frequency selective response. This provides an advantage of being an inexpensive and compact manner to provide the selective response.
When the circuit is used in applications where compactness is paramount, it is preferable that the circuit consumes relatively low power, thereby reducing a requirement for a bulky power supply or such a large battery for operating the circuit. The circuit can therefore incorporate a reflection amplifier for amplifying the portion of the received signal. Moreover, the storing means can be connected in a signal path between the antenna assembly and the reflection amplifier. This provides an advantage that the reflection amplifier is capable of providing a relatively high gain, for example in a range +10 to 30 dB, whilst consuming relatively low currents, for example a few microamperes.
Wideband reflection amplifiers providing gain and a response bandwidth of several GHz are capable of spontaneously oscillating, especially if they are imperfectly electrically matched to components connected thereto. The circuit advantageously therefore incorporates gain controlling means for switching the reflection amplifier in the reflector circuit alternately between a relatively more reflecting state and a relatively less reflecting state. This provides an advantage that gain provided by the amplifier is interruptible thereby counteracting a tendency for the amplifier to spontaneously oscillate.
Conveniently, the antenna assembly comprises a first antenna element for receiving the input radiation and a second antenna element for emitting the output radiation, said first and second antenna elements being mutually spatially separate. This provides an advantage that input and output from the circuit are isolated to a greater degree, thereby counteracting spontaneous oscillation within the circuit.
The antenna assembly preferably incorporates one or more of a patch antenna, a bow tie dipole antenna and a travelling wave antenna. These provide an advantage of being compact and suitable for use in various combinations at radio frequencies in a frequency range of several GHz.